Hollow latex particles as submicrometer reactors for polymerization in confined geometries

A method was developed for free‐radical polymerization in the confines of a hollow latex particle. Hollow particles were prepared via the dynamic swelling method from polystyrene seed and divinylbenzene and had hollows of 500–1000 nm. So that these hollow poly(divinylbenzene) particles could function as submicrometer reactors, the particles were filled with a monomer (N‐isopropylacrylamide) via the dispersion of the dried particles in the molten monomer. The monomer that was not contained in the hollows was removed by washing and gentle abrasion. Free‐radical polymerization was then initiated by γ radiolysis in the solid state. Transmission electron microscopy showed that poly(N‐isopropylacrylamide) formed in the hollow interior of the particles, which functioned as submicrometer reactors. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5706–5713, 2004     

(6-4)-photolyase activity requires a charge shift reaction

A model compound containing a thymine oxetane moiety linked to a flavin chromophore was investigated regarding (6-4)-photolyase activity. The need for a charge shift reaction was demonstrated by a detailed pH-dependent kinetic analysis.     

Photoinduced Electron Transfer Reactions of alpha-Cyclopropyl- and alpha-Epoxy Ketones. Tandem Fragmentation-Cyclization to Bi-, Tri-, and Spirocyclic Ketones

Reductive photoinduced electron transfer (PET) reactions have been performed with various bicyclic alpha-cyclopropyl-substituted ketones and tertiary amines. The reaction resulted in a regioselective cleavage of one cyclopropyl bond under formation of an exocyclic radical with an endocyclic enolate unit. In the case of bicyclic ketones with an unsaturated side chain, various bicyclic, spirocyclic, and tricyclic products are accessible via radical cyclization, depending on the position of the alkenyl substituent. In addition to triethylamine, N-silylated amines have also been used as electron donors, leading to a variety of compounds, among them are silylated fragmentation products, indicating that a proton is transferred from not only the amine radical cation but also the cationic silyl group. The intramolecular Paternó-Büchi reaction has also been studied for cyclopropane derivatives of the jasmone type leading to tetracyclic oxetanes. Finally, alpha-epoxy-substituted ketones have been investigated under PET conditions, yielding ring-opened products.     

Méthylène-indolines,indolénines et indoléniniums—XIII: L'hydroxy-16 déhydro-1 vincadifformine,intérmédiaire dans le réarrangement biomimétique de la vincadifformine en vincamine

The title compound is a crucial intermediate in the biomimetic conversion of vincadifformine 1 into vincamine 7. Its configuration at C-16 is established by a combination of chemical and spectroscopic evidence. Iodine oxidation converts 14 into the bridged lactam 18, thus proving a β configuration for the hydroxy group at C-16. The same reaction applied to vindoline 19 gives 21 identical with one of the compounds obtained by microbiological transformation of 19. The ¹³C NMR spectra of derivatives 3 and 8 (obtained by oxidation of vincadifformine) show that oxidation proceeds with introduction of the substituent at C-16, with a β-configuration. The alcohol 3 however, posesses a different conformation due to strong hydrogen bonding with N-4.     

Colloidal Analysis of Particles Extracted from Microalloyed Steels

Different colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field-flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high-quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed.     

Highly stretched single polymers: atomic-force-microscope experiments versus ab-initio theory

Experimental single-molecule stretching curves for three backbone architectures (single-stranded DNA, various types of peptides, polyvinylamine) are quantitatively compared with corresponding quantum-chemical (zero-temperature) ab-initio calculations in the high-force range of up to two nanonewtons. For high forces, quantitative agreement is obtained with the contour length of the polymers as the only fitting parameter. For smaller forces, the effects of chain fluctuations are accounted for by using recent theoretical results for the stretching response of a freely-rotating-chain model.     

Avalanche statistics of sand heaps

Large-scale computer simulations are presented to investigate the avalanche statistics of sandpiles using molecular dynamics. We show that different methods of measurement lead to contradictory conclusions, presumably due to avalanches not reaching the end of the experimental table.     

Exposure of [MnIII6CrIII]3+ single-molecule magnets to soft X-rays: The effect of the counterions on radiation stability

X-ray absorption spectroscopy studies of the [Mn^III₆Cr^III]³⁺ single-molecule magnet deposited as a microcrystalline layer on gold substrates are presented. The oxidation state of the manganese centers changes from Mn^III to Mn^II due to irradiation with soft X-rays. The influence of the charge-neutralizing anions on the stability of [Mn^III₆Cr^III]³⁺ against soft X-ray exposure is investigated for the different anions tetraphenylborate (BPh₄^?), lactate (C₃H₅O₃^?) and perchlorate (ClO₄^?). The exposure dependence of the radiation-induced reduction process is compared for [Mn^III₆Cr^III]³⁺ with the three different anions.